1. Introduction
Linear actuators comprising rotary stepper motors which transmit their rotational movement via a screw to a linear movement are popular positioning devices in automotive engineering e.g. for vehicle headlight adjustment mechanisms, due to their reliability, relatively simple manufacturability and low cost.
Linear actuators apply a force to their load in the direction of movement, that is, in the axial direction of the rotor. To counter this axial load a fixed thrust bearing is provided to support the rotor against the reaction force. Generally, the thrust bearing is directly fixed to a wall of the motor housing or is formed by a portion of the wall. The end of the shaft is either rounded or has a steel ball embedded in the end to contact the thrust plate with minimal friction.
For a vehicle headlamp adjuster, these linear actuators have to withstand high vibrations (axial and radial) and a broad temperature range (−40 to +120° C.). The linear positioning accuracy and stability should be high (<0.1 mm). Dynamic response should be in the order of 10-20 mm/s. In the position holding mode the linear actuator is preferably not excited (no electric current) but still has to resist axial forces comparable to the dynamic loads.
According to their nature, stepper motors are accelerating and decelerating with every step movement performed. The stronger the motor (necessary for a highly dynamic response) the higher the torque variations. In addition, a weakly damped rotor-stator system can oscillate around an equilibrium state at its eigenfrequencies (natural frequencies of the inertia—spring torque system). These effects lead to vibrations and noise as well as to instability (resonances).
The requirements listed above require a sophisticated actuator design. Such a design may require a high mechanical stability with reduced axial play and mechanical vibration damping.
2. Prior Art
EP 1 363 382 shows a linear actuator with a ball bearing centered on the rotor between two permanent magnet halves. This design outstandingly solves the problem of accurate radial positioning of the rotor relative to the stator. It is also fairly immune against bending/tilting of the axle as a result of radial forces. However, the accuracy of the linear output is limited by the axial play of the ball bearing. Furthermore, the separation of the rotor magnet increases the risk of angular misalignment between permanent magnet and stator poles.
DE102009000975A1 shows a linear actuator with a ball bearing centered on the rotor within a tubular magnet forming a part of the rotor. This design also solves the problem of accurate radial positioning of the rotor relative to the stator. However, there is no control over the axial play of the rotor which is dependent on the axial play of the bearing supporting the rotor. Thus axial positioning of the rotor is not tightly controlled.
U.S. Pat. No. 3,161,447 discloses a bearing arrangement for a rotatable shaft which acts as a thrust bearing to absorb axial loads and which is automatically adjusted to take up any axial play. The principle of this design is fine for the axial play reduction, but it does not allow an on-axis linear output. The radial alignment is accomplished through simple sleeve bearings, which yield higher radial tolerances and increased friction with radial loads.
U.S. Pat. No. 7,682,045 discloses a linear actuator where the rotor is axially and radially supported on the motor side (opposite to the linear output) by a bearing, which comprises a ball rigidly attached to the rotor and inserted into a cavity formed in the housing. An axial stop is pressed against the ball by means of a spring. The contact is essentially a single point contact. This has the significant disadvantage that the entire rotating part is only held at the very ends of the axle. Hence, radial accuracy in the air gap between rotor and stator is poorly satisfied. Radial loads or fabrication tolerances may result in a bent axle, which will generate noise and vibrations or even prevent rotation of the rotor. Furthermore, the invention is focused on minimizing bearing friction. It does not provide a well controlled, constant, non-zero friction over the given temperature range.
Friction in standard bearings is strongly temperature dependent related to the lubricant chosen, particularly at low temperatures. It can not be used as a constant friction brake for damping purposes.
Damping of oscillatory rotor behavior in electric motors can be accomplished through a lossy coupling with an additional inertia disc (e.g. elastomeric material as used in U.S. Pat. No. 4,800,306 or magnetic hysteresis as used in U.S. Pat. No. 4,049,985). The disadvantages include: more parts, more space, more weight, more cost, and reduced dynamic response due to higher total inertia.
Damping can also be approached with intelligent driver electronics. However, these are significantly more expensive. Many users prefer to use simple low-cost electronics and therefore need a very robust general-purpose actuator.
Hence there is a desire for a linear actuator with high mechanical stability.